I discovered that some (few) providers don't copy into the Sent folder those E-mails which are sent from mail clients through SMTP. They only copy them to the sent folder if you send them from their web interface, everything sent directly through SMTP with a client is not copied in any folder.
Is it a standard complying behaviour or is it a bad practice? I notice for instance GMail correctly saves those emails to both [Sent] folder and to [IMAP/Sent].
I suspect this behaviour is only aimed at selling IMAP services (which permit to create folders and copy emails inside them).
Edit : I just want some reference to what's the correct behaviour when a server receives a mail through SMTP. Discard it from its local sent folder or not